1. Field of the Invention
This invention relates to thermally conductive materials, more particularly to a thermally conductive silicone composition.
2. Description of Related Art
Recently, removing heat from heat generating electronic appliances comes to be more and more important. Electronic appliances such as electronic instruments, computers, or mobile phones generate heat when they are used. The heat generated from the electronic appliances will affect their normal operation. Generally, heat dissipating sheets are applied in electronic appliances, for example, by being interposed between a heat generating electronic component and a radiator. The heat dissipating sheet which conducts heat from the electronic component to the radiator must have high thermal conductivity in order to efficiently cool the electronic appliances as described above. However, a clearance inevitably exists between the heat dissipating sheet and the electronic component. As such, a thermal impedance can be formed between the electronic component and the heat dissipating sheet, thereby greatly reduces the thermally conductive efficiency of the heat dissipating sheet. In order to reduce the thermal impedance between the electronic component and the heat dissipating sheet, thermally conductive silicone compositions are applied in the heat dissipating sheet. Thermally conductive silicone compositions can be used to fill the clearance between the heat dissipating sheet and the electronic component, thereby reducing the thermal impedance. Conventional thermally conductive silicone compositions have low heat conductivities (e.g., in a range from 0.1 W/mK to 0.3 W/mK). Therefore, heat conducting fillers are applied to thermally conductive silicone compositions to improving their heat conductivities. Presently, as heat dissipating manners, thermally conductive silicone compositions can be made into heat dissipating sheets as set forth above, heat dissipating greases or phase change materials to meet various requirements.
A thermally conductive composition is disclosed in U.S. Pat. No. 5,981,641. The thermally conductive composition includes a liquid silicone base, an aluminum nitride (AlN) filler and a zinc oxide (ZnO) filler dispersed into the liquid silicone base. A particle diameter of the aluminum nitride is equal to that of the zinc oxide, and is in a range from 0.1 micrometers to 5 micrometers. A content of the zinc oxide in the sum of the aluminum nitride and the zinc oxide is in a range from 0.05% to 0.5% by weight. A content of the sum of the aluminum nitride and the zinc oxide in the thermally conductive composition is in a range from 83% to 91% by weight. A heat conductivity of the thermally conductive composition is in a range from 2.5 W/mK to 3.7 W/mK. In this thermally conductive composition, zinc oxide can improve a lubricative property of the particles of the aluminum nitride.
U.S. Pat. Nos. 6,114,429 and 6,162,849 disclose a thermally conductive composition. In the conductive composition, a mixture of two types of liquid state silicon emulsion is used as a base, and three types of fillers are dispersed into the base. The three types of fillers respectively are aluminum oxide particles with a particle diameter of 40 micrometers to 80 micrometers, aluminum nitride particles with a particle diameter of 0.5 micrometers to 5 micrometers, boron nitride particles with a particle diameter of 1 micrometer to 10 micrometers, and silicon carbide particles with a particle diameter of 0.4 micrometers to 10 micrometers or zinc oxide particles with a particle diameter of 0.2 micrometers to 5 micrometers. Three types of above fillers with fine and coarse particles are mixed into the base to form a desired thermally conductive composition. A content of the sum of the three types of fillers in the thermally conductive composition is in a range from 50% to 90% by weight. A heat conductivity of the thermally conductive composition is in a range from 1.2 W/mK to 2.9 W/mK.
U.S. Pat. Nos. 6,174,841 and 6,255,257 disclose a thermally conductive composition. In the thermally conductive composition, two or more fillers with different diameters are dispersed into various silicone bases with different viscosity. The thermally conductive composition includes a main filler and a minor filler. The main filler is aluminum nitride. The minor filler is selected from a group consisting of aluminum oxide, boron nitride, silicon carbide and zinc oxide. A content of the main filler in the thermally conductive composition is in a range from 50% to 95% by weight. An average diameter of the particles of the main filler is in a range from 0.5 micrometers to 25 micrometers. A content of the minor filler in the thermally conductive composition is in a range from 0% to 50% by weight. These minor fillers can enable the filler dispersed in the silicone base having a highest density, thereby improving a heat conductivity of the thermally conductive composition. The heat conductivity of the thermally conductive composition is in a range from 1 W/mK to 3.5 W/mK.
U.S. Pat. No. 6,372,337 discloses a thermally conductive composition including fillers and a silicone base penetrating into clearances among the particles of the fillers. The silicone base is formed by mixing five or more liquid state silicones. The fillers include a main filler and a minor filler. The main filler can be aluminum particles, and an average diameter thereof is in a range from 0.5 micrometers to 50 micrometers. The minor filler can be boron nitride particles or zinc oxide particles. An average diameter of the boron nitride particles is in a range from 1 micrometer to 5 micrometers. An average diameter of the zinc oxide particles is in a range from 0.2 micrometers to 5 micrometers. A content of the sum of the main filler and the minor filler in the thermally conductive composition is in a range from 50% to 90% by weight. A heat conductivity of the thermally conductive composition is in a range from 3.3 W/mK to 4.2 W/mK.
U.S. Pat. No. 6,649,258 discloses a thermally conductive composition including fillers and a silicone base penetrating into clearances among the particles of the fillers. The silicone base is formed by mixing four or more liquid state silicones. The fillers include a main filler and a minor filler. The main filler can be aluminum particles, and an average diameter thereof is in a range from 0.1 micrometers to 50 micrometers. The minor filler can be zinc oxide particles. A ratio of the aluminum particles to the zinc oxide particles is in a range from 1/1 to 10/1. A content of the sum of the aluminum particles to the zinc oxide particles in the thermally conductive composition is in a range from 80% to 92% by weight. A heat conductivity of the thermally conductive composition is in a range from 1.7 W/mK to 3.8 W/mK.
U.S. Pat. No. 6,828,369 discloses a thermally conductive composition including an organic polymer, and spherical or non-spherical aluminum oxide particles dispersed into the organic polymer. As a heat conducting filler, the spherical or non-spherical aluminum oxide particles are sufficiently dispersed into the organic polymer. When a content of the heat conducting filler in the thermally conductive composition is higher than 70% by volume, a heat conductivity of the thermally conductive composition is larger than 5.5 W/mK.
However, regarding the above-described patents, a manufacturing cost of the main fillers is relatively high. In addition, the proportion and diameter of the main fillers render the thermally conductive composition having a low mechanical strength, low surface evenness or high surface roughness. As such, the thermally conductive silicone composition is difficult to be compressed, and a thermal impedance between the heat generating appliances and the thermally conductive silicone composition is relatively high.
Therefore, a thermally conductive silicone composition having low manufacturing cost, high mechanical strength, low thermal impedance, and is easily to be compressed is desired.